This invention relates to a process for producing composites cobonded with a staged adhesive, and is particularly concerned with a process for cobonding and staging together a composite in the form of a repair patch and an adhesive applied directly thereto, to form a staged laminate which can be applied to a damaged area of a substrate, e.g. a damaged aircraft component, and the laminate cured by elevated temperature heating to bond the repair patch or laminate to the substrate.
A system and method for producing a composite particularly applicable for repairing damage to a structural component such as an aircraft component, is disclosed in U.S. Pat. No. 4,560,428 to Sherrick, et al. According to the patent, a patch to be integrally connected to the parent material and comprising a fiber reinforced component such as graphite impregnated with a thermosetting resin such as an epoxy resin, is partially cured, e.g. "B" staged. A layer or film of heat bondable adhesive such as a "B" staged epoxy resin adhesive, is placed between the patch and the parent material to be repaired. The entire assembly is then cured, e.g. employing a vacuum bag cure cycle, to cure the staged epoxy resin of the patch and of the adhesive layer to effect integral bonding of the patch to the parent material.
One disadvantage of this prior art method is that the staged adhesive as well as the staged composite or patch require storage separately prior to use, usually at reduced temperature, to avoid further reaction of the staged resin. Further, the stored adhesive usually requires defrosting and trimming prior to use and it is also necessary to prepare the patch surface for bonding with the adhesive. Another disadvantage is that if the staged adhesive employed in the above patent process is not devolatilized, the volatiles from the adhesive can penetrate back into the patched material during curing, and create a porous laminate. Another problem is maintaining bondline thickness during cure.
U.S. Pat. No. 4,357,193 to McGann, et al discloses a method of fabricating a composite wherein a laminate comprising a plurality of plies of fibrous material in an uncured resin such as graphite-epoxy is placed in a vacuum chamber and a partial vacuum is created in the chamber. The laminate is heated to within a temperature range wherein the resin matrix exhibits a viscosity which allows air migration through the resin matrix. The laminate is maintained within such temperature range for a time sufficient to expel entrapped air in the laminate.
It is accordingly an object of the present invention to provide a process which results in a high strength adhesive which in the staged condition exhibits prolonged room temperature storability, and which is an integral part of the secondarily bonded element, e.g. a repair patch.
Another object is to develop a process of the above type wherein the adhesive is devolatilized prior to bonding to a composite element, such as a repair patch.
A still further object is the provision of a staged laminate having a staged adhesive bonded thereto, and which is storable and ready for use to repair a damaged substrate, by curing.